1. Field of the Invention
This invention relates to supporting frames, in particular, removable supporting frames for thin ferromagnetic masks such as are useful in manufacturing processes.
2. Description of Related Art
Metal masks and stencils useful in many manufacturing processes typically have frames by which the mask or stencil is handled and which serves to keep the mask taut and crease free and storage devices to keep the mask flat and protected. Mask or stencil and frame typically expand at different rates during heating due to differences in coefficient of thermal expansion and/or differences in thermal mass. Some processes, most particularly that disclosed in U.S. Pat. No. 5,539,153 assigned to Hewlett-Packard Company provide for heating a mask, hereinafter referred to as Hewlett-Packard CPD process or CPD (for "contained paste deposition"). A frameless mask is desirable because during heating or cooling the mismatched expansion of rigid frames damage or destroy the mask. However, the thin metal mask must be kept flat and handled properly in order for the manufacturing process to be commercially feasible. A substitute for a rigid frame is necessary. Many difficulties are presented by rigid frames and alternative support devices or securing methods, such as vacuum. In applications where the mask is heated, a rigidly attached frame causes large tensile stresses to build up in the mask when the temperature of the mask/frame assembly changes. These stresses damage the mask.
Protruding mechanical fasteners can dent the mask when the mask is repeatedly loaded and unloaded from the frame or ring. Protruding fasteners also require alignment with holes in the mask during loading. Fasteners entrap liquids and cleaning solutions during cleaning. Thus, protruding fasteners reduce some of the benefit of a reusable mask. A manufacturing process solution is needed that maximizes the economy of a reusable mask.
Freestanding masks are also problematic as far as current solutions are concerned. The lack of a support ring causes the mask to bend out of plane during any handling step as well as shipping, cleaning and measuring of the mask. These bending stresses cause plastic deformations which permanently damage the mask.
Mask attachment by vacuum solves some problems but creates others. Vacuum attach to a mask support requires that the support be attached to a vacuum source. The requirement of a vacuum source renders vacuum attach impractical for remote manipulations such as shipping and storage. Moreover, vacuum during a cleaning process may suck water and cleaning agents into vacuum lines which can damage vacuum equipment, adding cost to the process.
Finally, sliding parts can create particles which cause defects in processes such as integrated circuit fabrication.
What is needed is a mask support that introduces little or no tensile stress, and that eliminates out-of-plane bending. What is also needed is a mask support that is easy to attach and detach, without protruding features or fasteners, and without vacuum or sliding parts. Further desirable is the elimination of drag-out of liquids during cleaning; such elimination ensures that this excess moisture does not adversely affect subsequent processes or mask.